This invention relates generally to pressure sensors for force-balance transmitters, and more particularly to a self-contained pressure sensing cell which may be coupled to the force beam of the transducer section of the transmitter and which may readily be replaced without disturbing this section.
Transmitters for measuring absolute, gauge or differential pressure which operate on the principles of force-balance are well known in the art. One commercially-available instrument of the force-balance type is the Pressure-to-Pneumatic Transmitter Series 50 PW 1000 manufactured by the Fischer & Porter Company of Warminster, Pa. and described in the 1978 Instruction Bulletin 50 PW 1000, Revision 2, published by this company.
In a transmitter of this type a Bourdon tube or bellows, depending on the pressures to be measured, acts to sense the process pressure. This sensor is directly coupled to one end of a pivoted force beam and produces a torque directly proportional to the applied pressure. An equal and opposite force from a feedback system is applied to the other end of the beam, this opposing force being generated by a feedback bellows that measures the transmitter output pressure.
When measuring differential pressure, a second Bourdon tube or bellows is used to sense the lesser of the two measured pressures. This second pressure sensor is mounted in opposition to the first sensor so that the force beam detects only the differential value. When measuring absolute pressure, the second sensor is evacuated and sealed, so that the pressure applied to the first sensor is compared to a zero reference rather than atmospheric pressure.
The above-described Series 50 PW 1000 instrument yields a pneumatic signal in the 3-15 PSIG range directly proportional to the measured process pressure. Transmitters are also commercially available which convert pressure into corresponding electrical values. One such instrument is the Series 50 EP 1000 transmitter also manufactured by Fischer & Porter and described in their 1978 Instruction Bulletin 50 EP 1000-C, Revision 5.
This electrical instrument also operates on force-balance principles to produce an output current in a useful industrial process control range (i.e., 4, to 20 mAdc or 10 to 50 mAdc). In this instrument, the opposing torque applied to the force beam is produced by a force motor that measures the transmitter's output current. U.S. Pat. Nos. 3,564,923 and 3,832,618 are representative of prior art force-balance transmitters of the electrical type, while U.S. Pat. No. 3,742,969 describes a force-balance transmitter of the pneumatic type.
The primary concern of the present invention is with a pressure sensor responsive to an input pressure to produce a corresponding force. Pressure sensors for force-balance transmitters which make use of conventional bellows or convoluted diaphragms have limited use in pressure applications over 1,000 psi. Their sizes are such that the forces generated by the sensors, as determined by their P/F ratio characteristics, are too high for the standard industrial type of force-balance transducer.
In one well-known differential-pressure transmitter manufactured and sold by Fischer & Porter of Warminster, Pa., and described in their Instruction Bulletin 50 DP 3000 Revision 3 (March 1978), two related pressure inputs are applied to high and low pressure chambers incorporated in the meter body. A convoluted sensing diaphragm in each chamber acts to sense the applied input pressure and to convert it to a force equal to the product of the pressure and the effective area of the diaphragm. These two forces are then projected in opposite directions to the lower end of the pivoted force beam through diaphragm pins. The resultant differential force generates a torque causing the force beam to swing through a relatively small angle about a sealing diaphragm functioning as a fulcrum.
In this Fischer & Porter transmitter, the two sensing diaphragms are welded to the meter body block to define a single internal chamber that is filled with a damping fluid. This fluid influences the dynamic response characteristics of the sensors and acts as an hydraulic back-up to protect the diaphragm under high static pressure.
As pointed out previously, convoluted diaphragm pressure sensors are unsuitable for very high psi pressure levels. But another drawback of such sensors is that they have a limited life, particularly when exposed to corrosive fluids. When it becomes necessary to replace a sensor in a differential pressure transmitter installation, this cannot be done in the field; for the diaphragms are welded to the meter body and the sensors are structurally integrated with the transducer section of the transmitter. Hence one must return the entire transmitter to the shop to replace the sensors, as a result of which the transmitter may be out of service for a prolonged period.
Bourdon tube pressure sensors are available which have P/F ratio characteristics suitable for high pressure applications up to psi levels as high as 10,000 to 20,000. But because of their physical shape, Bourdon tube sensors tend to trap impurities as well as the condensed moisture encountered in other than ideal process wetted applications. To eliminate these problems, chemical seals are often employed, but such sealing expedients add appreciably to the cost of the sensor.